Can't connect to IMAP-server

Hello!

Wanted to use the mail-plugin and did a manual setup. But I can’t connect to the server, the inbox remains empty. I used starttls on port 143.
In the menu lefthand there is the inbox shown and beneath the emailaddress with an exclamation mark.

Also in the serverlog I see Error: stat(…/.dovecot.sieve/tmp) failed: Not a directory

dovecot.sieve is a symlink to .sieve/managesieve.sieve
ls- l
drwx------ 2 vmail vmail 4096 14. Apr 2019 cur
-rw------- 1 vmail vmail 400 17. Aug 12:38 dovecot.index.log
-rw------- 1 vmail vmail 51 17. Aug 12:38 dovecot-uidlist
-rw------- 1 vmail vmail 860 5. Dez 12:51 managesieve.sieve
drwx------ 2 vmail vmail 4096 14. Apr 2019 new
drwx------ 2 vmail vmail 4096 13. Dez 15:47 tmp

What can I do to get the thing right?

Andreas